A new type of rechargeable lithium battery known as lithium-ion or `rocking chair` has recently become available commercially and represents a preferred rechargeable power source for many consumer electronics applications. These batteries have the greatest energy density (Wh/L) of presently available conventional rechargeable systems (ie. NiCd, NiMH, or lead acid batteries). Additionally, lithium ion batteries operate around 31/2 volts which is often sufficiently high such that a single cell can suffice for many electronics applications.
Lithium ion batteries use two different insertion compounds for the active cathode and anode materials. Insertion compounds are those that act as a host solid for the reversible insertion of guest atoms (in this case, lithium atoms). The excellent reversibility of this insertion makes such compounds function extremely well in rechargeable battery applications wherein thousands of battery cycles can be obtained. In a lithium ion battery, lithium is extracted from the anode material while lithium is concurrently inserted into the cathode on discharge of the battery. The reverse processes occur on recharge of the battery. Lithium atoms travel or "rock" from one electrode to the other as ions dissolved in a non-aqueous electrolyte with the associated electrons travelling in the circuit external to the battery. Although the insertion process is very reversible, a gradual loss of lithium and/or buildup of impedance still can occur upon extended cycling for various reasons. This in turn typically results in a gradual loss in delivered capacity with cycle number.
3.6 V lithium ion batteries based on LiCoO.sub.2 /pregraphitic carbon electrochemistry are now commercially available (eg. products of Sony Energy Tec. or A&T Battery). Many other lithium transition metal oxide compounds are suitable for use as the cathode material, including LiNiO.sub.2 (described in U.S. Pat. No. 4,302,518) and LiMn.sub.2 O.sub.4 (described in U.S. Pat. No. 4,507,371). Also, a wide range of carbonaceous compounds is suitable for use as the anode material, including coke (described in U.S. Pat. No. 4,702,977) and pure graphite (described in U.S. Pat. No. 4,423,125). The aforementioned products employ non-aqueous electrolytes comprising LiBF.sub.4 or LiPF.sub.6 salts and solvent mixtures of ethylene carbonate, propylene carbonate, diethyl carbonate, and the like. Again, numerous options for the choice of salts and/or solvents in such batteries are known to exist in the art.
P.sub.2 O.sub.5 is a common chemical and its properties are well known. It is known to decompose into various hydrogen-phosphorous-oxygen containing compounds in the presence of water. P.sub.2 O.sub.5 has been used extensively in the art as a reactant for preparing components in lithium batteries. For instance, the prior art contains numerous references to the use of P.sub.2 O.sub.5 as a crystallization modifier in the preparation of vanadium oxide cathode compounds for lithium metal anode batteries (see for example Journal of the Electrochemical Society, Vol. 135, No. 4, April 1988, p.791, Y. Sakurai et al.). In said preparation, the P.sub.2 O.sub.5 is a precursor and exists as alpha or beta VPO.sub.3 in the product cathode.
The prior art also contains references wherein P.sub.2 O.sub.5 is used in the preparation of other cathode compounds, but again the P.sub.2 O.sub.5 is substantially changed chemically during the preparation. For example, Mitsubishi Cable Industries in European patent application 571,858 describe the preparation of lithium-cobalt-phosphate cathode compounds and Sanyo in Japanese patent application laid-open no. 01-067869 describe the preparation of treated manganese oxide cathode compounds.
Additionally, the prior art contains references to the use of P.sub.2 O.sub.5 as a reactant in the preparation of anode compounds for lithium ion batteries. For instance, Sony in PCT Application WO 9216026 describe the preparation of phosphorous-carbon anode compounds. Again, the reactant P.sub.2 O.sub.5 is substantially modified chemically by the preparation.
Also, P.sub.2 O.sub.5 has been used in the art as a precursor for the preparation of certain glassy solid electrolytes (as in the preparation of an oxide/sulfide glass described in Proc. Electrochem. Soc., 91-12 (Proc. Int. Symp. Ionic Mixed Conduct. Ceram.), 145-54 (1991) by S. Jones et al. or the preparation of an oxide glass mix described in Solid State Ionics, 40-41, p680-3 (1990) by B. Chowdari et al.).
Thus, although P.sub.2 O.sub.5 has been used extensively as a reactant for components employed in non-aqueous lithium batteries, until recently P.sub.2 O.sub.5 itself seems not to have been identified as a useful battery component or additive.
In Canadian Patent Application Serial No. 2,150,877, by the same inventors, filed Jun. 2, 1995, it is demonstrated that exposing the electrolyte of certain non-aqueous rechargeable lithium batteries to P.sub.2 O.sub.5 can result in improved battery fade rate characteristics. (Fade rate was defined therein as the fractional loss of capacity per cycle.) This can be accomplished by incorporating the P.sub.2 O.sub.5 into either electrode. However, a simple means for exposure is to directly add P.sub.2 O.sub.5 particles to the electrolyte itself. The P.sub.2 O.sub.5 can be partly in solution or simply suspended in the electrolyte.
Incorporating P.sub.2 O.sub.5 in lithium batteries can improve the cycling performance of lithium batteries. A preferred method of incorporating for purposes of mass production is to partially dissolve and/or suspend P.sub.2 O.sub.5 powder in a liquid electrolyte. However, with certain desired electrolyte formulations, such incorporation can result in a substantial undesirable increase in viscosity of the electrolyte. The viscosity can become increasingly non-newtonian and exhibit the characteristics of a pseudoplastic. (The term pseudoplastic describes behaviour wherein the viscosity increases with decreasing shear rate. Such behaviour is common for suspensions or slurries generally.) Unfortunately, a substantial increase in viscosity, particularly at low shear rates, poses a problem when such electrolytes are used in the manufacture of lithium batteries.
Typically, during lithium battery manufacture, electrolyte is introduced after the dry assembly of the internal components (including electrodes, separators, current collectors, etc.) Although the dry assembly components may actually be fairly porous, it is nonetheless difficult to get common non-aqueous electrolytes to permeate the microporous network of the dry assembly. In order to accelerate the electrolyte filling operation, it is common to use pressure differentials created by vacuum and/or ambient pressure exposures to assist the filling. Higher viscosity electrolytes slow down the filling process even more, or conversely require even greater pressure differentials. Neither situation is desirable.